Radiation-induced solid-state polymerization of derivatives of methacrylic acid. IV. Electron spin resonance spectra of barium methacrylate dihydrate

The esr spectrum of γ-irradiated barium methacrylate dihydrate at -196°, attributed to the radical (CH)ĊCOO, was observed to change to the spectrum characteristic of the propagating radical RCHĊ(CH)COO on warming above -100°. This transformation was temperature and time dependent. The spectrum of the propagating radical after several days at -80° consisted of 9 lines with hfs of 11.5 G and an increasing intensity distribution. Immediately on warming to 50° the spectrum was similar, but on cooling back to -80° an alternating intensity distribution was observed, and further cooling to -154° resulted in splitting of the intermediate lines; this effect was reversible. When maintained at 50°, the intensities of the intermediate lines decreased continuously so that the increasing intensity distribution changed to an alternating intensity distribution and finally to a 5-line spectrum. These different spectra are all attributed to the methacrylate propagating radical and are considered to result from different conformations of the β-methylene carbon-hydrogen bonds. The conformation of the radical, and hence the esr spectrum, is believed to depend on (1) the length of the radical chain, (2) the temperature, and (3) the environment. Various models are considered to explain the changes in conformation